It is known that a metal strip can be cast by continuous casting in a twin roll caster. Molten metal is introduced between a pair of counter-rotating horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the region where the rolls are closest together, referred to as the nip, between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The molten metal may be introduced into the nip between the rolls via a tundish and a metal delivery nozzle located beneath the tundish so as to receive a flow of metal from the tundish and to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls. To make sheet metal, alloys are cast directly into strips to reduce the amount of hot rolling that thicker products require. In this process, a high rate of heat extraction from the strip to the rolls is desired to maximize productivity. At the same time, heat extraction must be uniform over the entire surface area of the strip to minimize stresses that induce surface cracks.
However, the interface between the rolls and the metal is the dominant resistance to heat flow (see for example Y. Kim, B. Farouk, and J. Keverian: Journal of Engineering for Industry, 1991, vol. 113, pp. 53–58) and many defects are produced here during the initial contact between the molten metal and the mold (see for example H. Yasunaka, K. Taniguchi, M. Kokita, and T. Inoue: Iron Steel Inst. Jpn. Int., 1995, vol. 35, pp. 784–89). The condition of the rolls that act as the mold is determinant in the characteristics of this interface and may need to be tailored to the specific needs of the alloy that is cast. In particular, the temperature of the water cooled rolls is a feature of the process that may be properly adjusted to assist in achieving the desired solidification. The rotation of the rolls produces temperature cycles with a rise when contact of the solidifying metal with the rolls prevails and a fall when it ceases. For stainless steel strips solidifying on copper rolls, the surface temperature has been cited to cycle between approximately 200 to 400° C. (see for example F. Macci and A. Mollo: La Revue de Métallurgie-CIT, 1995, vol. 92, pp. 789–794). With steel rolls, the temperature cycles for the same alloy were between 400 and 800° C. (see for example F. Macci and A. Mollo: La Revue de Métallurgie-CIT, 1995, vol. 92, pp. 789–794). Tests performed by some of the present inventors using a prototype caster to produce carbon steel strip on nickel plated copper rolls yielded subsurface temperature cycles between 100 and 200° C. Temperature cycles have also been observed in single roll strip casting (see for example G. Li and B. G. Thomas: Met. Mat. Trans., 1996, vol. 27B, pp. 509–525).
Several patents have addressed the issue of heat transfer intensity and uniformity in strip casting. U.S. Pat. No. 4,887,662 issued to Tanaka et al. teaches that improvement in surface quality is accomplished by reducing the heat extraction capacity of the rolls. This decrease in productivity is achieved by machining dimples (0.1–1.2 mm in diameter and 5–100 μm deep) at the surface of the rolls. The slow cooling approach to improve surface quality is also disclosed in U.S. Pat. No. 5,103,895, issued to Furuya et al. and U.S. Pat. No. 5,964,277 issued to Tanaka et al. which offer alternative methods to reduce heat extraction.
Other patents disclose methods of increasing the heat transfer capacity of the rolls to increase productivity. In U.S. Pat. No. 5,701,948 issued to Strezov et al. grooves of specific depth and spacing are machined at the surface of the rolls. However, designing dimples, grooves or any other specific pattern at the surface of the rolls requires vigilant maintenance since dimensional changes occur through usage. In U.S. Pat. No. 5,720,336 issued to Strezov it is claimed that an infiltration of a thin layer, less than 5 μm, of molten oxide at the interface between the rolls and the strip will improve the overall interfacial contact and hence heat transfer. However, with the infiltration of a molten oxide at the interface between the rolls and the strip, the composition and the thickness of that layer must be carefully controlled. Since the layer is likely made in-situ from chemical elements found in steel, this method is difficult to apply to the wide range of compositions found in commercial steels and generally cannot be applied to non-ferrous alloys.
The prior art does not discuss the effects of these methods on the surface quality of strips, which depends more on the uniformity of the heat flow than its intensity. To the present, the effect of the roll temperature on the heat transfer and the surface quality of strips has not been reported. In addition, the prior art does not address the issue of simultaneously increasing heat transfer intensity and uniformity.